A range of optical flow cells, or so-called liquid core waveguides have been developed for optical spectroscopy applications in the ultraviolet, visible and infrared regions of the light spectra. Such flow cells are particularly suitable when combined with optical fibers for light transfer, enabling the design of a flexible sensor system. A number of flow cells having a long optical pathlength have been designed for absorbance, fluorescence and Raman spectroscopy. Similar to optical fibers, light is confined in such tubular flow cells within the (liquid) core by total internal reflection at the liquid core/wall interface or the liquid core/cladding (coating) interface. The liquid core typically comprises a sample solution. The refractive index of the cell wall or cladding must be lower than the refractive index of the liquid core.
Flow cells can generally be divided into two types (Type 1 and Type 2) on the basis of the light guiding effect and practical observations. In a Type 1 flow cell a polymer tubing works as cladding or wall of the flow cell containing the sample liquid (core) of the flow cell. In Type 1 flow cells the sample liquid (core) is in direct contact with the cladding. Thus the cladding must have a lower refractive index than the 1.33 refractive index of a typical water based sample solution. A Type 2 flow cell comprises a glass or polymer capillary tubing coated at the outside surface with a low refractive index polymer. The sample liquid (core) is contained within the capillary tubing. In Type 2 flow cells the capillary is a transparent high-refractive index layer separating the low refractive index cladding material from direct contact with the core fluid. Light is coupled into the liquid sample core and travels through the capillary wall, which does not interfere with the waveguide properties of the cell. The cladding must have a lower refractive index than e.g., the 1.33 refractive index of a water based sample solution.
For an optical coating used in a liquid core waveguide application, coating thicknesses must be a minimum of 5 times the highest optical wavelength used to ensure light guidance. The highest optical wavelength used can typically be 1000 nm, requiring an optical coating thickness equaling 5 micrometers or more. This becomes especially important when working with samples of different refractive indices, where the light guidance in the waveguide changes as a function of the refractive index.
Very thin optical coatings of about 3000 angstroms can be made by physical vapor deposition from bulk TEFLON AF. However, it is desirable to provide optical coatings without the need for expensive vapor deposition equipment. It is also desirable to provide optical coatings having a greater thickness than is typically available from vapor deposition.
The early development of waveguide sample cell technology was made difficult by the absence of a suitable cladding material, which possessed a refractive index lower than that of the water based sample solution (n=1.33), a most commonly used solvent. This problem was originally solved by Schwab et al and later by Tsunoda et al. who used a bar quartz capillary suspended in air. In these arrangements, light would be reflected at the outer air/glass interface. However, light transmission was found to be strongly dependent on the cleanliness of the external cell surface. Ambient dust and fingerprint contamination of the exterior cell surface could easily degrade light transmission and thus the reproducibility of the analytical measurements. Tiny cracks could develop at the external surface resulting in a brittle, easily broken capillary cell.